The present disclosure relates generally to over voltage protection for power converters.
Power converters, which usually supply power to appliances used in daily life, need to be equipped with protection mechanism to prevent abnormal satiations from damaging users or surroundings. For example, a power converter that powers light emitting diodes for lighting must have over-voltage protection (OVP) so as to avoid over voltage occurring in its outputs, which might cause electric shock to human beings if touched.
FIG. 1 demonstrates a conventional power converter 10. Bridge rectifier 12 provides full-wave rectification to alternative-current (AC) mains voltage VAC to generate rectified direct-current (DC) input voltage VIN and a ground line. Power converter 10 is a buck converter having LED module 14 as a load, which connects in series with a primary winding PRM in a transformer, between DC input voltage VIN and the ground line. Power controller 17 has a power switch 18, which, when turned on (as being in a conduction state), energizes primary winding PRM and conducts a driving current to illuminate LED module 14. When power switch 18 is turned off (as being in a non-conduction state), primary winding PRM starts to release its stored energy to generate another driving current, which passes wheel diode 16 to keep LED module 14 illuminating. Current-sense resistor 20 provides to power controller 17 current-sense signal VCS, a representative of the current passing through power switch 18.
If a LED open event happens to LED module 14, meaning that at least one LED in LED module 14 is open or cannot conduct current, driving voltage VLED could rocket if there is no corresponding protection mechanism, or OVP, built in power controller 17. The two end terminals of LED module 14, which meanwhile has a drop voltage the same with the rocket-high driving voltage VLED, could cause severe electric shock to anyone whoever touches them, endangering human beings.
Power controller 17 in FIG. 1 detects driving voltage VLED, through the help from the combination of node VOP, voltage divider 22 and secondary winding SEC. When the transformer de-energizes to release its stored energy, the voltage across primary winding PRM is about the summation of driving voltage VLED and the forward voltage of wheel diode 16, and the voltage across secondary winding SEC is in proportion to that across primary winding PRM. Accordingly, in case that the voltage at node VOP exceeds a certain limit when the transforming de-energizes, it implies driving voltage VLED is somehow over high, and, responsively, power controller 17 could continuously turn off power switch 18 to stop power conversion of power converter 10, achieving OVP.